Ultrasonic flowmeters are commonly used to determine the flow rate for a variety of fluids (e.g., liquids, gases) flowing in pipes. Knowledge of the flow rate of the fluid can enable other physical properties or qualities of the fluid to be determined. For example, in some custody-transfer applications, the flow rate can be used to determine the volume (Q) of a fluid (e.g., oil or gas) being transferred from a seller to a buyer through a pipe to determine the cost for the transaction, where the fluid volume is equal to the flow rate multiplied by the cross-sectional area of the pipe and the time duration of interest.
Non-invasive clamp-on flow monitors for pipes are known, such as for water flow metering. A non-invasive flow monitor can be clamped to the outside of a pipe and secured thereto, using appropriate brackets and fasteners.
Invasive inline flow monitors for pipes are also known which are mounted within an intervening pipe section that joins to the adjacent pipe sections by a flange. One type of ultrasonic flowmeter employs transit time flow metering, where one or more pairs of ultrasonic transducers are attached to a pipe (or a spool piece attached to a pipeline), where each transducer pair includes a transducer located upstream with respect to the fluid flow and a transducer located downstream with respect to the fluid flow. Each transducer, when energized, transmits an ultrasonic beam or signal (e.g., a sound wave) along an ultrasonic path through the flowing fluid that is received by and is detected by the other transducer of the transducer pair. The path velocity (i.e., path or chord velocity (Vp)) of the fluid averaged along an ultrasonic path can be determined as a function of the transit time differential between the transit time of an ultrasonic signal traveling along the ultrasonic path from the downstream transducer to the upstream transducer, and the transit time of an ultrasonic signal traveling along the ultrasonic path from the upstream transducer to the downstream transducer.
There are two different measurement principles used in known transit time ultrasonic flowmeters. A first type of ultrasonic flowmeter is a direct path type that implements direct measuring crossed paths between transducer (sensor) pairs, where there are no reflectors needed. The ultrasonic transmitter and receiver for the direct-path type ultrasonic flowmeter are located in a linear configuration within the fluid flowing inside the meter pipe. A second type of ultrasonic flowmeter is a reflective path type that implements indirect measuring paths generally using at least one ultrasonic reflector mounted on the meter pipe inner wall opposite to the transducer pair to reflect the ultrasonic measurement signal received from the ultrasonic transmitter to the ultrasonic receiver, where the transducer pair is located at the same side of the meter pipe wall.
In operation, a pulse train excitation is generally used to excite one transducer of the transducer pair. A conventional way to process the received ultrasonic signal resulting from the pulse train excitation is to compute the zero crossings of the received signal from which the difference in transit time (or delta time of flight, ΔTOF) between the upstream and downstream paths is calculated, which is used to compute the fluid flow.